1. Field of the Invention
The invention relates to a rim for a cycle wheel, and a method for manufacturing such a rim.
2. Background Information
Conventionally, a cycle wheel rim is annular and has a U-shaped radial cross section, with two lateral sidewalls that are generally perpendicular to the axis of rotation of the wheel and a bridge that connects the lateral sidewalls and is oriented so as to be opposite the axis of rotation of the wheel. In the case of a spoked wheel, the spokes are generally fastened to the bridge of the rim.
To manufacture a cycle wheel rim, it is known to make a rectilinear section by extruding or spinning a metal alloy, such as an aluminum alloy, for example, and then bending the section into an annular shape. The ends of the rim are then connected to one another, for example by welding.
It is advantageous to produce a lightweight rim, in order to optimize the performance of the cyclist, while maintaining sufficient rigidity and strength to avoid breakage.
The extrusion process requires minimum wall thicknesses, for the yield stress increases significantly when the thickness decreases and the characteristics of the alloy and the spinning speed increase. Thus, for alloys that are typically used in the manufacture of rims, it is very risky to go below thicknesses of 0.85 mm, as doing so may quickly break the extrusion die.
In addition, the bending of a section causes significant plastic deformations, particularly in the zones that are away from the center of gravity of the cross section. These substantial plastic deformations cause dislocations of the grains of the material which, in turn, can result in a surface finish known as “orange peel,” which is often considered unsightly.
Furthermore, overly thin lateral walls also generate local buckling of the walls of the section during bending, when subject to overly high compressive deformations.
To overcome the aforementioned drawbacks, the document FR-A-2 727 355 proposes to make a rim by extruding a rectilinear section having sufficient thickness to prevent the extrusion from causing defects in the material. The section is then bent, and it does not change and does not buckle due to its thickness. To reduce the weight of the rim, the rim is then immersed in an aggressive chemical bath which attacks the material and decreases its thickness. This manufacturing method is relatively cumbersome to implement and, although the surface finish does become uniform, it remains quite rough.
Similarly, the document U.S. Pat. No. 6,961,999 proposes extruding and bending a sufficiently thick section in order not to cause manufacturing defects. The lateral sidewalls of the rim are then machined to reduce their thickness and to decrease the weight of the rim. The document EP-A-0 579 525 also proposes machining the lateral sidewalls of the rim after extrusion and bending of the section. Reducing the weight of the rim is not optimal because the bridge of the rim retains its initial thickness.
It is also known, from the documents EP-A-1 084 868 and EP-A-1 491 362, to machine the section after bending, both in the area of the bridge and in the area of the lateral sidewalls of the rim, and only on certain portions of the circumference of the rim. The machining is time-intensive and it is complex to implement, because it requires the use of a five-axis machine. When it is desired to produce a plurality of rims having various geometries, for example with respect to the number of spokes, the location of the spoke fastening zones, or the thickness of the rim, all of these methods require a modification of the manufacturing parameters, in particular with respect to the machining for material removal and/or chemical milling. This leads to additional development costs and does not standardize the manufacturing method.